1. Field of the Invention
The present invention relates to a power transmission device that transmits power when a normal load is applied and interrupts the power transmission when an excessive load is applied.
2. Description of the Related Art
In a compressor for an automobile air conditioner, to which power is typically transmitted from an external power source, such as an engine, etc., via a belt, if the compressor seizes up, excessive torque load may occur and damage the engine, etc. Therefore, in order to interrupt the power transmission in such case, a power transmission device having a torque limiter is used.
FIG. 8 shows a power transmission device 11 as described above that has: a belt pulley 13 around which a belt and the like is wound; a hub 15 that is fitted and secured to this belt pulley 13 via an elastic member; a power interruption member 17 that is inserted and secured inside this hub 15; and a rotating shaft 19 that is screwed inside this power interruption member 17.
The rotating shaft 19 has: a rotating shaft body 21; a right hand male thread section 23 that is provided at a tip of this rotating shaft body 21 which has a diameter smaller than that of the rotating shaft body 21; and a stepped surface 25 that is formed between this male thread section 23 and the rotating shaft body 21.
The hub 15 has a contact surface 27 thereinside that makes contact with the rotating shaft body 21 via the stepped surface 25 and a washer 35.
The power interruption member 17 has: an interruption member body 29 that is secured to the hub 15; a tubular member 31 that is screwed to the male thread section 23 of the rotating shaft 19; and a narrow diameter section 33 that integrally couples this tubular member 31 with the interruption member body 29. Here, reference numeral 37 denotes a compressor that is driven by the rotating shaft 19.
In the power transmission device 11 described above, when a normal load is applied between the belt pulley 13 and the rotating shaft 19, due to the fact that the female thread section of the power interruption member and the male thread section of the rotating shaft 19 are screwed to each other, the contact surface 27 of the hub 15 is pressed against the stepped surface 25 of the rotating shaft 19 via the washer 35. This pressing force allows the hub 15 to be connected with the rotating shaft 19 in a static friction state, so that a driving force of the belt pulley 13 is transmitted to the rotating shaft 19 to drive the compressor.
During operation under a normal load, when the rotating shaft 19 is locked due to seize-up of the compressor and the like, this impact allows the static friction state to turn into a dynamic friction state, so that the contact surface 27 of the hub 15 rotates with respect to the stepped section 25 of the rotating shaft 19. Due to this rotation, the tubular member 31 of the power interruption member 17 also rotates with respect to the male thread section 23 of the rotating shaft 19, and as a result, the tubular member 31 is pulled by a large force in a direction away from the interruption member body 29 and the narrow diameter section 33 ruptures. Therefore, the power transmission between the belt pulley 13 and the rotating shaft 19 is interrupted, so that breakage of the belt and resulting damage of the engine can be prevented.
The torque limiter as described above has advantages that it is not sensitive to fatigue, etc., but it has a problem that variation of working torque at which power transmission between the belt pulley 13 and the rotating shaft 19 is interrupted is excessive.
The working torque is determined by a coefficient of friction between thread surfaces, and dimensional accuracy and material strength of the narrow diameter section. In particular, the material strength of the narrow diameter section varies significantly depending on production lots and manufacturers. Therefore, a tolerance range of the working torque of the torque limiter has to be set rather wide, for example, to 50-120 Nm and the like.
A lower limit of the tolerance range of the working torque has to be larger than the maximum torque under normal operation of the compressor, however, in order to protect the driving belt and prevent engine stall, its upper limit has to be smaller than a torque value at which the belt slips and the engine stalls.
In addition, with reference to FIGS. 4, 4a and 4b, the spacer 93 serves as a first abutment section that faces toward the rotating shaft body 47and abuts against the stepped surface 65, which may be referred to herein as a first stepped section. The hub 75, or wheel, includes the first inner circumferential surface 103 and a second stepped section 103a formed between the first inner circumferential surface 103 and a second inner circumferential surface 103b. The second inner circumferential surface 103b has a diameter larger than that of the first inner circumferential surface 103.
Further, with reference to FIGS. 4, 4a and 4b, the tubular member 81 includes the first outer circumferential surface 105 and a second abutment section 105a, which is formed between the first outer circumferential surface 105 and a second outer circumferential surface 105b. The first outer circumferential surface 105 is press-fitted into the first inner circumferential surface 103, and the second abutment section abuts against the second stepped section.
However, in recent years, measures for cost reduction, such as reduction of the number of pistons and the like in the compressor are taken, and as a result, fluctuations in driving torque tend to increase. On the other hand, tension on the driving belt is reduced for fuel consumption, and as a result, belt slip tends to decrease.
If the tolerance range of the working torque cannot be set narrow, it is impossible to incorporate the torque limiter into an automobile. Therefore, it is desirable to implement a torque limiter in which the variation of the working torque is reduced.
Other examples of the torque limiter described above are shown in Japanese Unexamined Patent Publications No. 2003-206950 and 2003-307265.